1. Field of the Invention
The present invention relates to blocked polyisocyanates prepared by blocking partially trimerized cyclic organic diisocyanates having (cyclo)aliphatically bound isocyanate groups, to one-component coating compositions containing this blocked polyisocyanate and a polyhydroxyl component and to the coatings obtained therefrom.
2. Description of the Prior Art
There is a need in the automotive industry for a clear topcoat which can be applied over existing basecoats and which provides improved environmental etch resistance. The thermoset melamine/acrylics which are conventionally used as the clearcoat suffer from poor resistance to acid rain, bird droppings, tree sap, etc.
Recently, two-component polyurethane coatings have increasingly been used as clearcoats. These coatings possess excellent environmental etch resistance and also possess many other excellent properties such as appearance, durability, hardness and flexibility. However, the two-component polyurethane coating compositions suffer from one major disadvantage. They require two-component spray equipment as opposed to the conventional thermoset melamine/acrylics which are applied using one-component equipment. Therefore, an additional capital expenditure is required to obtain the necessary spray equipment for applying the two-component polyurethane coating compositions.
Accordingly, it is an object of the present invention to provide a one-component system which overcomes the disadvantages of the two-component polyurethane coating compositions.
This object may be achieved in accordance with the present invention by the use of the blocked polyisocyanates described hereinafter. The fact that these blocked polyisocyanates may be used for production of coatings with improved environmental etch resistance is surprising because the polyisocyanates used for the blocking reaction contain unreacted monomer. It would be expected that the presence of monomer, which lowers the average functionality of the polyisocyanate, would reduce the amount of crosslinking and result in coatings with reduced environmental etch resistance.